This invention relates to polymer emulsion compositions. More particularly, it relates to novel aqueous polymer emulsions of outstanding durability especially when utilized as pigment vehicles over chalked surfaces as compared with previously known polymer emulsions of similar ilk.
Aqueous dispersions or emulsions of synthetic polymers, e.g., homopolymers and copolymers of ethylenically unsaturated monomers such as vinyl esters, lower alkyl acrylates, lower alkyl methacrylates, and the like, have been known for many years. The procedures used to prepare such polymer emulsions generally involve adding, with rapid stirring, one or more polymerizable monomers to water, to which is added, either simultaneously or subsequently, surfactants or emulsifying agents, a polymerization catalyst or initiator and, in many cases, one or more plasticizers and protective colloids or colloid-forming substances. This system is usually heated, either before or during addition of the aforesaid ingredients, to a temperature at which polymerization will be fostered as well as controlled. The reaction system is then maintained at the required temperature and will substantially complete polymerization of the monomer charge as occurred. The resulting polymer emulsion, upon cooling and appropriate work-up, can be used in many domestic and industrial applications, such as in paints or in other coating compositions (e.g., paper coatings and textile treating compositions), in additives or binders, in caulking compositions, and the like, depending on the particular monomers involved and the properties of the emulsion properties derived therefrom.
Vinyl and acrylic polymer emulsions, particularly those derived from vinyl esters of carboxylic acids (e.g. vinyl acetate) and/or alkyl esters of alpha,beta-ethylenically unsaturated carboxylic acids (e.g., lower alkyl acrylates and lower alkyl methacrylates), have come to be used in ever-increasing amounts in recent years in the so-called waterbased or "latex" paints. These latex paints, being substantially odorless, non-flammable and non-irritating to the skin, are useful not only as household paints but also in industrial applications. With the development of new polymers, tailor-made for this use, aqueous polymeric coating compositions or latexes have been made with properties equal to or even superior to those possessed by some solvent-based paints. For example, typical latexes have excellent color-retention an outdoor exposure.
However, one undesirable feature associated with conventional oil-base and alkyd paints is chaulking, which occurs on exterior exposure and is primarily the result of a combination of weathering and ultraviolet degradation of the polymer binder. The chaulked surface has a definite water demand. This results in a strong wicking force which rapidly extracts the water from new latex paint, causing the film to form on the surface rather than penetrate to the substrate. Although a polymer latex is a dispersion of resin particles of micron size, the extremely small interstices of the chaulked surface resist penetration and act as a filter which lets water through but holds the solid resin particles back, thus preventing sound adhesion. As a result, while common latex paints do not chaulk on outdoor exposure, their adhesion to chaulky surfaces is very poor, and thus extensive surface preparation is necessary before they can be applied over chaulked paint films.
One expedient which has been used to permit the use of latex paints over chaulked paint films without prior surface preparation involves emulsifying the latex paint with rather substantial amounts, i.e., usually up to about 25% and sometimes as much as 50% by weight, based on the emulsion solids, of various additives such as solvents, oils, alkyds, and other proprietary specialties. Alkyds are polyesters of a polyhydric alcohol (e.g., glycerol or pentaerythritol) and a polybasic acid or anhydride (e.g., phthalic anhydride) modified with a fatty acid or oil. Upon application, the above-mentioned additives, which are merely physically present in the latex and form an essentially incompatible mixture with it, go out of the emulsion, penetrate the chaulky surface, and possibly bond with some oil or oil-like molecules from the original oil-base of the alkyd paint which are found at the surface of the old paint film and beneath chaulky layer. This can result in the formation of a matrix for the chaulky residue, which would then permit the latex portion of the emulsion to adhere to the painted surface.
Whatever the mechanism of action of these drying oil/alkyd/etc.-latex paint emulsions, their use gives rise to as many problems as it solves. Thus, for example, such emulsions themselves can chaulk, due to the presence of the various additives. They also have poor color retention. Additionally, such paint films harden to become brittle and yellow with age. Certain oils such as soya oil, in addition, suffer from very slow dry time and, when added, seriously impair the freeze-thaw stability of the latex. The alkyds as a class tend to be of higher molecular weight and, therefore, exhibit much higher viscosities than the oils. As a result, their penetration ability is reduced considerably. They also tend to reduce scrubability and impair freeze-thaw stability. In essence, then, the use of these additives physically emulsified with latex emulsions detracts from the very properties that are expected of the final latex product.
Significant gains have been made in recent years in eliminating the severity of many of the property problems above illustrated when the aforementioned additives were physically emulsified with the latex paints by chemically combining with the vinyl alkanoate and alkyl acrylate monomers a drying oil, for example, safflower oil, castor oil, oiticica oil, sunflower oil, soybean oil, perrilla oil, tall oil, dehydrated castor oil, poppy oil, tung oil, very long oil alkyds, long oil alkyds and the like. When this chemical union is accomplished, as taught in U.S. Pat. No. 3,547,847, as opposed to the physical emulsion techniques utilized in earlier art improvements are realized in certain properties of the latex paints that use these polymers as vehicles. Although the adhesion characteristics of these formulated paints over chaulky substrates are improved over the compositions wherein the dry oils and/or additives are physically emulsified, they still suffer from the severe disadvantage of realizing premature failure to adhere to chaulky paint surfaces due to their inability to adequately penetrate said surfaces.
Accordingly, it is an object of the present invention to provide a synthetic polymer which, when formulated as an aqueous emulsion, possesses superior adhesion characteristics over chaulky substrates.
Furthermore, another object is to provide superior latex paint vehicles using chemically combined acrylic modified alkyds.